The present invention relates to a fuel cell supplied with liquid fuel, and more particularly, to a fuel cell suitable for size reduction.
The fuel cell is an electric power generation device which generates electric power by supply of fuel and oxidizer. Since air is generally used as the oxidizer, the fuel cell is capable of continuous power generation by replacement of a fuel container. Therefore, the fuel cell has been drawing great attention not only as stationary power supply but also as portable power supply.
In stationary fuel cells, hydrogen gas or a gas containing hydrogen is generally used as a fuel. For the portable power supply, however, it is advantageous to be capable of generating electric power for longer time by using the fuel stored in an identical size container, and therefore, liquid fuels which are higher in energy density per volume are more advantageous than gas.
For power generation, it is also possible to use hydrogen which a reformer generates from a liquid fuel. However, the reformer complicates the whole fuel cell system, and it is considered that direct supply of liquid fuel easily achieves size reduction of the fuel cell system.
As an example of the fuel-direct-supply type fuel cell, a direct methanol fuel cell has been conventionally disclosed (JP 11-510311 A, JP 6-188008 A), wherein a mixture of methanol and water is used as fuel.
A typical direct methanol fuel cell is explained with reference to FIG. 6.
The direct methanol fuel cell is provided with a fuel electrode 103, an oxidizer electrode 102 and an electrolyte membrane 101 in a housing 116, as shown in FIG. 6. Firstly, mixture fuel of methanol and water is fed from a fuel tank (not shown) to a fuel introduction passage 108 by using a feeding means such as a feed pump. The fuel, which is fed to the fuel introduction passage 108, is then fed to a fuel electrode chamber 106 formed in a separator 105 along a direction shown by an arrow 110. The fuel, which is fed into the fuel electrode chamber 106, penetrates into the fuel electrode 103 and reacts so as to generate protons (hydrogen ions), electrons and carbon dioxide. A porous material is normally used for the fuel electrode 103, so that the reaction of the fuel electrode 103 takes place on a layer thereof in proximity to the interface with the electrolyte membrane 101, which layer carries a catalyst.
The protons, which are generated in the fuel electrode 103, move from the fuel electrode 103, permeate the electrolyte membrane 101, and move to the oxidizer electrode 102. The electrons flow from the fuel electrode 103 via an external circuit (not shown) to the oxidizer electrode 102. These electrons are used as an output of the cell. Meanwhile, the carbon dioxide is discharged from the fuel electrode 103 through the fuel electrode chamber 106, along with the unreacted fuel.
Also, oxygen is fed to an oxidizer electrode chamber 104 formed in the separator 105 along a direction shown by an arrow 111. The oxygen is then diffused from the oxidizer electrode chamber 104 into the oxidizer electrode 102 and reacts with the protons that have diffused from the fuel electrode 103 so as to generate water. The generated water, which is normally in the form of water vapor, and the unreacted oxygen are discharged from an exit of the oxidizer electrode chamber 104 in a direction shown by an arrow 112. Oxygen is used as an oxidizer in this example. Air may also be used although its oxygen concentration is lower.
It should be noted that, in the above conventional direct methanol fuel cell, carbon dioxide discharged from the fuel electrode chamber 106 and gas emitted from the oxidizer electrode chamber 104 have to be discharged independently of each other. This is because the separator 105 separates the fuel electrode chamber 106 from the oxidizer electrode chamber 104, as shown in FIG. 6. Accordingly, in this conventional fuel cell, an exhaust means such as a pump is needed for each of the oxidizer electrode chamber 104 and the fuel electrode chamber 106, which becomes an obstacle to size reduction. Eliminating a pump or other forcible exhaust means provided for exhaust of carbon dioxide in the fuel electrode chamber 106 would cause the efficiency in carbon dioxide exhaust to be lower, which causes a power decrease.